… I look at this through the lens of “sectoral financial balances”, an analytical framework learned from the work of the late Wynne Godley. The essential idea is that since income has to equal expenditure for the economy, as a whole, (which is the same things as saying that saving equals investment) so the sums of the difference between income and expenditures of each of the sectors of the economy must also be zero. These differences can also be described as “financial balances”. Thus, if a sector is spending less than its income it must be accumulating (net) claims on other sectors.

The crucial point is that, since sectoral balances must sum to zero, a rise in the deficit of one sector must be matched by an offsetting change in the others. It follows that if the fiscal deficit is increasing, the sum of the surpluses of the other sectors of the economy must be increasing in a precisely offsetting manner.

These are tautologies. But the virtue of this framework is that it forces us to ask what drives what: are, for example, fiscal deficits in the US (or UK) driving the surpluses in other sectors or are the surpluses in the other sectors driving the fiscal deficit? We can obtain answers by examining what behaviour is changing…

and that:

… The idea that the huge fiscal deficits of recent years have been the result of decisions taken by the current administration is nonsense. No fiscal policy changes explain the collapse into massive fiscal deficit between 2007 and 2009, because there was none of any importance. The collapse is explained by the massive shift of the private sector from financial deficit into surplus or, in other words, from boom to bust…

The sectoral balances approach should always be handled with supreme care. There are causalities running in all directions and one needs to ask what brings them to equivalence, what the value of policy instruments are, how is output changing etc.

Although the three balances must always sum to exactly zero, no single balance is more a residual than either of the other two. Each balance has a life of its own, and it is the level of real output that, with minor qualifications, brings about their equivalence. Underlying the main conclusions of our reports is an econometric model in which exports, imports, taxes, and private expenditure are determined as functions of such things as world trade, relative prices, tax rates, and flows of net lending to the private sector. However, neither the knowledge that this is the case nor the perusal of any list of econometric equations will, on its own, impart any intuition as to why output moved as it did over any set period.

It is well known to students of the National Accounts that the surplus of private disposable income over expenditure is equal to the government balance (written as a deficit) plus the current balance of payments (written as a surplus). While these balances are related to one another by a system of accounting identities, each has, to some extent, a life of its own that is reconciled with the other two via the aggregate income flow. The way the balances evolve provides a useful armature around which to organise a narrative account of economic developments, because any one of them is necessarily implied by the other two. Furthermore, the balances may give an early warning that unsustainable processes are taking place, for any high or rising balance implies a change in public, private, or foreign debts, which cannot grow without limit relative to income.

Wynne Godley with his CEPG partner Francis Cripps (from Cambridge Group Sings The Blues, The Guardian, 17 April 1980)